Organic polymers (plastics) and in particular polyolefins such as polyethylene and polypropylene require the addition of various additive systems thereto in order both to be processed and to retain long term stability in order to retain desired service properties. In addition to the damaging influence of light and heat, residues of the catalyst system used in the production of such plastics are also detrimental. To overcome such difficulties a wide variety of substances are known in the art for use as additives and stabilizers. In many instances a mixture of such additives is employed.
One commonly used additive system is that comprising a sterically hindered phenol antioxidant which is employed either alone or in combination with an organic phosphite and/or an acid neutralizer. Since such antioxidants are in the powder form, there is presented a disadvantage in the use of same in an additive system due to the problem of dusting as well as having a tendency toward separation and proving difficult to meter. Thus there exists a need for a commercial form of antioxidant additives which does not have these disadvantages.
While a variety of approaches have been made to achieve the production of a low dust additive system, such process have generally employed systems that introduce into the additive package a further component such as calcium stearate, water or other binders. Such a system is that disclosed in U.S. Pat. No. 5,597,857 wherein at least 10 percent by weight of calcium stearate is employed as a binding agent in the formation of additive granules.
Other prior art systems using mixing processes or compacting using a pellet press are known but in most cases the resulting commercial forms do not have adequate mechanical properties.
In addition to an additive package or pellet having adequate mechanical strength or hardness so as to have sufficient abrasion resistance to preclude dust formation, such a pellet needs also to have a balanced hardness which will permit it to be readily processed in the systems wherein the additive package is being dispersed into the host plastic.
Accordingly, there remains a need for a balanced strength antioxidant additive system which possess adequate mechanical strength to avoid mechanical abrasion and dust formation while at the same time having a balanced hardness which will permit it to be readily dispersed in the host plastic while at the same time avoiding the introduction of undesired components.
It is thus an object of the present invention to provide a process for the production of a novel pellet of a sterically hindered phenol antioxidant which will avoid the introduction of undesired components into the additive system.
Another object of this invention is to provide a novel sterically hindered phenol antioxidant system in a pellet form which will have a balanced hardness so as to provide adequate abrasion resistance yet being readily dispersed in a host plastic.
Other aspects, objects and the several advantages of this invention will become apparent from the following specification and claims.